The 3,000-metre thick Greenland icecap is a key concern in debates about climate change because a total melt would raise world sea levels by about 7 metres. And a runaway thaw might slow the Gulf Stream that keeps the North Atlantic region warm.

Glaciers at sea level have been retreating fast because of a warming climate, making many other scientists believe the entire icecap is thinning.

But satellite measurements showed that more snowfall is falling and thickening the icecap, especially at high altitudes, say Johannessen and team.

"The overall ice thickness changes are ... approximately plus 5 centimetres a year or 54 centimetres over 11 years."

But, they say, the thickening seems consistent with theories of global warming, blamed by most experts on a build-up of heat-trapping gases from burning fossil fuels in power plants, factories and cars.

Warmer air, even if it is still below freezing, can carry more moisture. That extra moisture falls as snow below 0°C.

And the scientists say that the thickening of the icecap might be offset by a melting of glaciers around the fringes of Greenland. Satellite data is not good enough to measure the melt nearer sea level.

Ice sheets

Most models of global warming indicate that the Greenland ice might melt within thousands of years if warming continues.

Oceans would rise by about 70 metres if the far bigger icecap on Antarctica melted along with Greenland.

Antarctica's vast size acts as a deep freeze likely to slow any melt of the southern continent.

The panel that advises the United Nations has predicted that global sea levels might rise by almost a metre by 2100 because of a warming climate.

Such a rise would swamp low-lying Pacific islands and warming could trigger more hurricanes, droughts, spread deserts and drive thousands of species to extinction.

Separate study supports sea level rise due to ice melt

A separate study in today's issue of Science reports that sea levels are probably rising slightly because of a melt of ice sheets.

"Ice sheets now appear to be contributing modestly to sea level rise because warming has increased mass loss from coastal areas more than warming has increased mass gain from enhanced snowfall in cold central regions," the report by a team led by Professor Richard Alley of Pennsylvania State University in the US says.

"Greenland presently makes the largest contribution to sea level rise."